Liquid crystals are used for displays in various technologies. A liquid crystal operates by electrically controlling an orientation of a special liquid crystal material. The orientation affects the intensity of the light passing through the liquid crystal. A liquid crystal cell is often built by sandwiching liquid crystal materials between a reflective electrode and a transparent top plate. The voltage on the electrode is changed to modulate the intensity of the light which is reflected from the electrode, and thereby change the effective gray level of the cell. An Mxc3x97N active matrix can be formed using individual cells of this type. The voltage level on the electrodes is changed correspondingly to change the image that is displayed by the liquid crystal.
The electrodes in the cells can be driven through a pass gate, such as an NMOS or CMOS pass gate. The analog level modulates the liquid crystal. However, since the total number of cells can be large, not all of the cells are driven simultaneously. With K input signals, the active matrix can be accessed sequentially K cells at a time. A cell needs to hold its voltage value between the times when it is driven. A sample and hold circuit can be used in each cell. Sampling is done by switching the NMOS pass gate. The value is conventionally held by associating a capacitor with the electrode.
Leakage across the capacitor causes the voltage on the capacitor to drop over time. If the display has 256 grayscales, the capacitor may need to be refreshed before its voltage drops by {fraction (1/512)} or about 0.2 percent. This necessitates relatively large capacitors and a relatively high refresh frequency. Such a system is called an analog modulated silicon light modulator or SLM.
The polarity of driving the liquid crystal material should also be alternated to prevent the LC material from becoming permanently rotated. Systems often invert the voltage between the top plate and the electrode during odd cycles.